Interpretive Summary: thProgrammed cell death (PCD), as the name implies, is a genetic program in all living organisms that allows certain cells to die at a specific time and place in the body to allow the normal development of an organism. Failed PCD can have serious consequences including disease and abnormal development. A collaborative study between scientists from University of Ljubljana, Slovenia and USDA, ARS scientists at the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, FL led to an unexpected discovery of PCD that occurs during the seed development in maize. The PCD was manifested at a very early stage by the total disintegration of nuclei, the center of all genetic information in the cell; however, the empty cell corpses remained in place throughout the duration of seed development. Further, our results suggest that such cellular suicide was aimed at facilitating these 'dead' cells to perform an important function of transporting water and nutrients from the mother plant to a developing seed. It also appears that this type of PCD in seed development may have potential in breeding for drought resistance. Further research includes a survey of corn germplasm and sorghum for PCD in relation to water stress during seed development.

Technical Abstract:
We present cellular level studies here to show programmed cell death (PCD) of placento-chalazal (PC) cell layers of maternal pedicel tissue in developing caryopses of normal seed, Mn1, and in the invertase-deficient miniature (mn1) seed mutant in maize. PCD was evidenced by loss of nuclei and all sub-cellular membranous organizations in many PC cell layers. The TUNEL stain that is diagnostic of apoptoptic-like PCD identified spatially and temporally two distinctive sub-domains within the PC cells. The early wave of PCD was TUNEL-negative and was specific to only the fertilized caryopses, indicating that the signaling for enucleation in these maternal cells originated in the zygotic tissues. In fact, the initiation of enucleation coincided with endosperm cellularization and was rapidly and coordinately completed in several files of the PC layer prior to the beginning of the major storage phase in endosperm. Cell shape in the PC layers also appeared to be influenced by the genotype of filial endosperm. The later wave of PCD was restricted to the files underneath the enucleated cells, and was TUNEL-positive in both genotypes. Further, the two sub-domains of early and late PCD cells were characterized by different sets of cell wall associated flavonoids. Based on collective evidence we infer that the early PCD may have osmolytic etiology and may lead to activation of the post-phloem transport function of the PC layer;whereas, the second wave of PCD may be senescent-related, in particular, protecting the maturing seed against microbes that may be transported from the maternal tissue.